“Wang Qishan is one of the most important figures in Xi’s inner circle, and this position allows him to retain a formal position,” said Wu Qiang, a former lecturer in political science at Tsinghua University in Beijing who now works as an independent researcher.

“Even in the United States, the vice president is usually ceremonial, there just as backup,” Mr. Wu said. “But Wang Qishan will add substance to the role of vice president. The amendment of the Constitution has raised the status of the presidency, and the vice presidency will also benefit from that.”

Party insiders and experts have said Mr. Xi wants Mr. Wang to act as a counselor, possibly helping to watch economic policy and anticorruption efforts and manage ties with the West, especially with the United States. President Trump has considered placing stiff trade sanctions and investment restrictions on China.

Mr. Wang appears likely to “play a leading role in overseeing U.S.-China relations,” said Ryan Hass, a former director for China at the National Security Council who is now a fellow at the Brookings Institution. Mr. Wang will work alongside Yang Jiechi, the former Chinese Foreign Ministry official who last year was promoted to the Politburo, a council of 25 senior party members, Mr. Hass said.

“Wang will operate at a more strategic level, in theory to help keep the relationship from going off of the rails,” Mr. Hass said.

But Mr. Wang could face a potent rival within the Chinese government for influence over trade policy toward the United States: Liu He, a longtime economic adviser to Mr. Xi who is expected to be named on Monday by the National People’s Congress as one of the country’s four vice premiers.

Mr. Wang will bring longstanding bonds with American politicians and business leaders to the task. His ties to Wall Street executives include John L. Thornton, a former president of Goldman Sachs, who last year helped arrange a meeting between Mr. Wang and Stephen K. Bannon, Mr. Trump’s former chief strategist. Mr. Wang also helped to steer trade and investment talks with Washington.

“He is the man China’s leaders look to for an understanding of the markets and the global economy,” Henry M. Paulson Jr., who was Treasury secretary under President George W. Bush and knew Mr. Wang well, wrote in 2009. “He is a Chinese patriot, but he understands the U.S.”

But Chinese officials have felt frustrated in their attempts to negotiate with Mr. Trump’s White House. There is no sure evidence that Mr. Wang can succeed, especially if Vice President Mike Pence does not emerge as his counterpart to manage tensions, Mr. Hass said.

“Trump really values being the person in the cockpit steering U.S.-China relations,” Mr. Hass said. “So I think Wang Qishan’s impact on the relationship remains an open question.”

While many National People’s Congress delegates welcomed Mr. Wang’s elevation, his election was also a sign of how Mr. Xi has recast political conventions so that he can consolidate power and surround himself with trusted supporters. For five years, from late 2012, Mr. Wang was in charge of the party’s discipline commission, from which he pursued Mr. Xi’s withering drive against corruption and political laxity.

Party sources said last year that Mr. Xi had raised the idea of keeping Mr. Wang in the formal party leadership, which would have bent an unspoken rule that top officials step down if they have reached age 68 when the congress meets to appoint new leadership. Instead, Mr. Wang, now 69, left his party positions at that congress and largely disappeared from public view in the following months.

“I think Wang will play an important role in determining whether Xi succeeds over the next five years and can smoothly make a transition to a third term,” Mr. Wu, the analyst in Beijing, said. “Xi thinks there are many challenges still ahead, I believe, so Wang has been kept on as kind of a consultant.”

Mr. Xi and Mr. Wang first met about five decades ago during the Cultural Revolution, when they were both sent from Beijing to work as “sent down youth” in rural northwest China, a poor and dusty corner of the country. Mr. Wang lived on a commune about 50 miles from Mr. Xi, who has said Mr. Wang stopped by for a night and took away a book on economics.

After China began to loosen controls on the economy from the late 1970s, Mr. Wang left his job as a historian in a state institute and became an expert on rural reforms, zipping around Beijing on a Japanese motorbike, then a rare sight in Beijing.

Mr. Wang accumulated experience in economic policy, and from the 1990s he rose through a series of increasingly senior government jobs, defusing financial messes and getting to know financiers on Wall Street. In 2003 he was made mayor of Beijing to deal with SARS, or severe acute respiratory syndrome, a virus that spread after officials hid the epidemic.

As deputy premier for five years from 2008, Mr. Wang was a key negotiator in trade and investment talks with the United States. After the global financial crisis erupted, Mr. Zhang also took charge of a group of officials assigned with designing China’s response.

But Mr. Wang is no economic liberal.

“You were my teacher,” Mr. Wang told Mr. Paulson, the former Treasury secretary recalled in his memoir. “We aren’t sure we should be learning from you anymore.”